This invention relates to the field of prosthetics, and more particularly, to an intervertebral disc prosthesis designed to replace a damaged intervertebral disc.
The human spine consists of twenty-four small bones known as vertebrae, or “vertebral bodies,” that protect the spinal cord and provide stability to the torso. The vertebrae are arranged in a column and stacked vertically upon each other. Between each vertebra is a fibrous bundle of tissue called an intervertebral disc. These intervertebral discs act as a cushion to the spinal column by absorbing energy and transmitting loads associated with everyday movement. They also prevent the vertebrae from rubbing against each other.
Each intervertebral disc comprises two distinct regions. A firm outer region, the annulus, maintains the shape of the intervertebral disc. An inner region, the nucleus, provides a resilient tissue that enables the disc to function as a shock absorber. Over time, the normal aging process causes the intervertebral discs to degenerate, diminishing their water content and thereby reducing their ability to properly absorb the impact associated with spinal movements. Diminished water content in the intervertebral discs may also cause the vertebrae to move closer together. Tears and scar tissue can weaken the discs, resulting in injury. When the discs wear out or are otherwise injured, a condition known as degenerative disc disease results. With this condition, discs do not function normally and may cause pain and limit activity.
The condition of degenerative disc disease can potentially be relieved by a surgical procedure called artificial disc replacement. In this procedure, the damaged intervertebral disc is replaced by a prosthetic disc. One well known intervertebral prosthetic disc is produced by DePuy Spine, Inc. of Raynaham, Mass. and is sold under the trademark CHARITÉ®. This disc prosthesis is comprised of two metal endplates and a center polyethylene core. The center core includes a superior spherical bearing surface and an inferior spherical bearing surface. The superior endplate includes a concave surface that fits upon and is congruent with the superior bearing surface of the core. The inferior endplate includes a concave surface that fits under and is congruent with the inferior bearing surface of the core. During the CHARITÉ® artificial disc replacement procedure, the damaged disc is first removed via an anterior surgical approach and the end surfaces of the exposed vertebrae are cleared of debris. The vertebrae are spread apart and the metal endplates are positioned on the respective vertebra and tapped into place. The polyethylene core is then inserted between the endplates and the vertebrae are returned to their normal position. The pressure of the spinal column further seats the endplates into the vertebral bones and secures the core in place.
Although current intervertebral disc prosthetic devices have enjoyed success, it would be beneficial to add additional desirable features to the prosthetic device. For example, it would be desirable to design the prosthetic device to more closely mimic the true anatomy and physiology of an intact intervertebral disc. Additionally, it would be advantageous to allow for alternate means of fixation of the implant to the surrounding bone. Furthermore, the implant could be improved by designing an instrument interface that is more compact, enhances ease of insertion, and accommodates non-anterior surgical approaches and/or revision procedures. For example, it would be desirable to provide feature that would allow the intervertebral disc prosthesis to be implanted in the patient as a unitary assembled piece rather than sequential component-by-component implantation. In addition, it would be advantageous to design the intervertebral disc prosthesis such that the implant materials and core geometry are modular, and can be selected to suit specific patient and/or surgeon needs. For example, for a patient with nickel hyper-sensitivity, non-sensitizing implant materials could be selected. Similarly, for surgeons that desire MRI compatibility, non-ferromagnetic materials could be selected.